1. Field of the Invention
The present invention relates to a steel radial tire for large-sized vehicles such as a bus or a truck and, more particularly to a steel radial tire having excellent durability and a steel cord with excellent corrosion resistance and fatigue resistance.
2. Description of the Related Art
The steel radial tire is a combination of material characteristics possessed by steel filaments and rubber. Particularly from the viewpoint of enhancing the product workability and durability of the tire, the rate of steel filaments relative to rubber is determined considering balance between the rigidity of steel filaments and the softness of rubber.
U.S. Pat. No. 3,726,078 discloses the technique of using steel cords for the carcass, belt and breaker of the tire.
U.S. Pat. No. 4,158,946 discloses a steel cord having a three-layer strand structure (3+9+15), as shown in FIG. 1. In the case of the steel cord of this three-layer strand structure, however, the space between the filaments becomes too large and many portions in the steel cord are left not filled with rubber. This makes it liable that the steel filaments are separated from rubber. Namely, the so-called separating phenomenon is liable to be caused.
Japanese Utility Model Disclosure Sho 63-186798 discloses a steel cord having a two-layer strand structure in which steel filaments each having a same diameter are used, as shown in FIGS. 2 and 3. In the case of this two-layer strand structure (12+n), the filling of rubber between the strands can be improved but the amount of rubber filled into the center core is not enough.
The bonding of steel filaments relative to rubber depends upon the surface condition of each of the steel filaments and the condition of rubber filled. When tires of a vehicle are repeatedly compressed, stretched and bent while they are in use (or the vehicle is running) in the case where these steel cords whose core portion is not fully filled with rubber are used for the tires, therefore, three steel filaments in the core portion of each cord are moved in rubber and worn while rubbing against one another. In short, the so-called fretting is caused. When this fretting is caused, the durability of the tires becomes remarkably low.
Particularly in the case of the steel cord disclosed by Japanese Utility Model Disclosure Sho 63-186798, nine steel filaments in the inner strand are closely contacted with one another and this makes it difficult for rubber to penetrate into three steel filaments in the core. When a needle, for example, sticks into the tire and its front end reaches the steel cords in the tire, therefore, water can penetrate into the tire along the needle for a short time and move through those clearances in the steel cord which are not filled with rubber. As the result, the steel cord corrodes.
Further, when the grasping force of steel filaments in the outer and inner strands relative to three steel filaments in the core portion is small, ends of these three steel filaments in the core portion come out of the tire surface, which can cause an accident.